1. Field of the Invention
This invention relates in general to semiconductor memories and, more specifically, to devices and methods for margin testing a semiconductor memory by, for example, equilibrating all complementary and true digit lines of the memory to ground simultaneously during a test mode.
2. State of the Art
As shown in FIG. 1, one conventional method for margin testing a sub-array 10 of a semiconductor memory begins with storing a supply voltage V.sub.CC on all memory cell capacitors 12 of the sub-array 10. This is accomplished by writing logical "1" bits to memory cells attached to true digit lines D0, D1, etc. using sense amplifiers 14 and even row lines R0, R2, etc., and by writing logical "0" bits to memory cells attached to complementary digit lines D0*, D1*, etc. using the sense amplifiers 14 and odd row lines R1, R3, etc. A cell plate voltage DVC2 equal to one-half the supply voltage V.sub.CC is applied to the cell plate of each memory cell capacitor 12.
Once each memory cell capacitor 12 has stored the supply voltage V.sub.CC, the row line R0, for example, is fired, causing the memory cells attached to the row line R0 to dump their stored charge from their memory cell capacitors 12 onto the true digit lines D0, D1, etc. This causes the sense amplifiers 14 to pull each of the true digit lines D0, D1, etc. up to the supply voltage V.sub.CC, and to pull each of the complementary digit lines D0*, D1*, etc. down to ground. As a result, a fill V.sub.CC -to-ground voltage drop is imposed across NMOS access devices 16 of the memory cells attached to the complementary digit lines D0*, D1*, etc. The V.sub.CC -to-ground voltage drop is maintained across the NMOS access devices 16 for a predetermined refresh interval of typically about 150 to 200 milliseconds (ms). This stresses any "leaky" NMOS access devices 16 and causes any such NMOS access devices 16 to lose significant charge from their memory cell capacitors 12 to the complementary digit lines D0*, D1*, etc. to which they are attached.
Once the predetermined refresh interval is over, all of the memory cells attached to the complementary digit lines D0*, D1*, etc. are read. Any of these cells that read out a logical "1" bit as a result of leaking excessive charge, instead of reading out the logical "0" bit they originally stored, are flagged as failing the margin test.
Once the memory cells attached to the complementary digit lines D0*, D1*, etc. have been tested, the memory cells attached to the true digit lines D0, D1, etc. are tested by firing the row line R1, for example. This causes the memory cells attached to the row line R1 to dump their stored charge from their memory cell capacitors 12 onto the complementary digit lines D0*, D1*, etc. In turn, this causes the sense amplifiers 14 to pull each of the complementary digit lines D0*, D1*, etc. up to the supply voltage V.sub.CC, and to pull each of the true digit lines D0, D1, etc. down to ground. As a result, a full V.sub.CC -to-ground voltage drop is imposed across NMOS access devices 18 of the memory cells attached to the true digit lines D0, D1, etc. The V.sub.CC -to-ground voltage drop is maintained across the NMOS access devices 18 for another predetermined refresh interval of about 150 to 200 ms. This stresses any "leaky" NMOS access devices 18 and causes any such NMOS access devices 18 to lose significant charge from their memory cell capacitors 12 to the true digit lines D0, D1, etc. to which they are attached.
Once the predetermined refresh interval is over, all of the memory cells attached to the true digit lines D0, D1, etc. are read. Any of these cells that read out a logical "0" bit as a result of leaking excessive charge, instead of reading out the logical "1" bit they originally stored, are flagged as failing the margin test.
This conventional margin testing method thus typically takes two predetermined refresh intervals of about 150 to 200 ms. each to complete. Since row lines in different sub-arrays in a semiconductor memory typically cannot be fired simultaneously because the addressing of the row lines is local to their respective sub-arrays, this conventional method cannot be used on more than one sub-array at a time. As a result, in a semiconductor memory containing four sub-arrays, for example, the conventional method described above takes approximately 1.2 to 1.6 seconds to complete. Because of the large number of semiconductor memories that typically require margin testing during production, it would be desirable to find a margin testing method that can be completed more quickly than the method described above.
As shown in FIG. 2, another conventional method for margin testing a semiconductor memory has been devised to conduct margin tests more quickly than the method described above. In this method, a sense amplifier 20 includes equilibrating NMOS transistors 22 which equilibrate true and complementary digit line pairs D0, D0*, etc. to a bias voltage V.sub.BIAS on an equilibrate bias node 23 in response to an equilibrate signal EQ. It should be understood that an "equilibrate bias node" is a node to which a cell plate is coupled, and from which a bias voltage is globally distributed for use by equilibrating transistors throughout a semiconductor memory.
When the semiconductor memory is not in a margin test mode, a test mode signal TESTMODE is inactive, so that a PMOS transistor 24 is on and the bias voltage V.sub.BIAS on the equilibrate bias node 23 is equal to the cell plate voltage DVC2 on the cell plate 25. When the semiconductor memory is in a margin test mode, the test mode signal TESTMODE is active so that the PMOS transistor 24 isolates the equilibrate bias node 23 from the cell plate 25, and so an NMOS transistor 26 connects the equilibrate bias node 23 to a probe pad 28 positioned on the exterior of the semiconductor memory. A stressing voltage, such as ground, can then be applied to the probe pad 28 during margin testing to simultaneously stress memory cells attached to both the true and complementary digit lines D0, D0*, etc.
Because the method described immediately above does not require the firing of any row lines in order to stress memory cells, all cells in a semiconductor memory can be stressed at once using this method. As a result, this method only requires one predetermined refresh period to complete testing, no matter how many sub-arrays a semiconductor memory contains. Thus, the method dramatically improves the speed with which margin testing can be completed.
Unfortunately, the method described above with respect to FIG. 2 has proven difficult to implement because the PMOS transistor 24 generally does not reliably isolate the equilibrate bias node 23 from the cell plate 25. In addition, the probe pad 28 has proven to be a cumbersome addition to the exterior of a semiconductor memory.
Therefore, there is a need in the art for a device and method for margin testing a semiconductor memory that avoid the problems associated with the probe pad method described above while still providing a rapid margin testing device and method.